Self-suspending proppants for use in carbon dioxide-based fracturing fluids and methods of making and use thereof

ABSTRACT

Self-suspending proppants including proppant particles coated with a CO2-philic coating are provided. The CO2-philic coating may be lightly crosslinked and may have a physical structure that constrains CO2 molecules. Methods of making self-suspending proppants may include coating a proppant particle with a polymerizable precursor material of a CO2-philic material and polymerizing the polymerizable precursor material to form a self-suspending proppant are also provided. Additionally, hydraulic fracturing fluids that may include a CO2-based fluid and the self-suspending proppants and methods of treating subterranean formations by contacting a subterranean formation with hydraulic fracturing fluid and propagating at least one subterranean fracture are provided.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to U.S. ProvisionalApplication Ser. No. 62/458,132, filed Feb. 13, 2017, which isincorporated by reference in its entirety.

BACKGROUND Field

Embodiments of the present disclosure generally relate toself-suspending proppant systems. Specifically, embodiments of thepresent disclosure relate to self-suspending proppants and methods ofmaking the same, and carbon dioxide-based hydraulic fracturing fluidsand methods of using the same to treat subterranean formations.

Technical Background

Carbon dioxide (CO₂) may be used in hydraulic fracturing fluids toprovide non-aqueous alternatives to conventional water-based fluids, assome subterranean formations may be sensitive to water. Moreover,‘water-less’ fluids are more environmentally sound, minimizing depletionof natural-source freshwater often consumed in fracturing applications.However, conventional CO₂-based fluids are not sufficiently viscous tosuspend propping agents, such as “proppants,” that are added tofracturing fluids to hold open subterranean fractures during andfollowing fracturing treatment. Because of this reduced ability tosuspend proppants, CO₂-based fluids have not been widely considered foruse in fracturing fluids.

Accordingly, a need exists for non-aqueous hydraulic fracturing fluidsthat adequately support and suspend proppant particles. Historically, ithas been very difficult to thicken CO₂-based fluids using additives, asCO₂ is not a good solvent for high molecular-weight polymers. Thoughwater-based fluids may be easily viscosified owing to the many availablethickening agents that readily dissolve in water, CO₂-based fluids arenot easily viscosified, owing to a lack of available materials that areCO₂-soluble.

SUMMARY

Some embodiments of the present disclosure are directed toself-suspending proppants that include proppant particles coated with aCO₂-philic coating. The CO₂-philic coating is lightly crosslinked andhas a physical structure that constrains or is solvated by CO₂molecules.

Further embodiments of the present disclosure include hydraulicfracturing fluids composed of a CO₂-based fluid and a self-suspendingproppant. The self-suspending proppant is a proppant particle coatedwith a CO₂-philic coating. The CO₂-philic coating is lightly crosslinkedand has a physical structure that constrains CO₂ molecules.

Still further embodiments of the present disclosure include methods ofproducing self-suspending proppants. The methods of producingself-suspending proppants include coating a proppant particle with apolymerizable precursor material that includes a CO₂-philic component toform a coated proppant, and polymerizing the polymerizable precursormaterial to form the self-suspending proppant. The self-suspendingproppant is a proppant particle coated with a CO₂-philic coating. TheCO₂-philic coating is lightly crosslinked and has a physical structurethat constrains CO₂ molecules.

Still further embodiments of the present disclosure include methods fortreating subterranean formations. The methods for treating subterraneanformations include contacting a subterranean formation with a hydraulicfracturing fluid comprising self-suspending proppant particles, andpropagating at least one subterranean fracture in the subterraneanformation. The hydraulic fracturing fluids composed of a CO₂-based fluidand a self-suspending proppant. The self-suspending proppant is aproppant particle coated with a CO₂-philic coating. The CO₂-philiccoating is lightly crosslinked and has a physical structure thatconstrains CO₂ molecules.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of specific embodiments of thepresent disclosure can be best understood when read in conjunction withthe following drawings.

FIG. 1 is a schematic view of a proppant, a coated proppant, and aself-suspending proppant, according to embodiments shown and described.

FIG. 2A is a schematic view of a hydraulic fracturing fluid comprisinguncoated proppants.

FIG. 2B is a schematic view of a hydraulic fracturing fluid according toembodiments shown and described.

FIG. 3 is a schematic view of a subterranean fracture being contactedwith a hydraulic fracturing fluid according to embodiments shown anddescribed.

DETAILED DESCRIPTION

Embodiments of the present disclosure are directed to self-suspendingproppants that include proppant particles coated with a CO₂-philiccoating. The CO₂-philic coating may be lightly crosslinked and may havea physical structure that constrains CO₂ molecules. The proppantparticle coated with the CO₂-philic coating may be suspendable inCO₂-based fluids such as hydraulic fracturing fluids, for example. Thehydraulic fracturing fluids may be used in methods for treatingsubterranean formations. Embodiments also include methods of making theself-suspending proppants and of making hydraulic fracturing fluidscomprising self-suspending proppants. Further embodiments includemethods of using hydraulic fracturing fluids to treat a subterraneanformation.

Subterranean formations such as rock, coal, or shale are treated bypumping a hydraulic fracturing fluid containing proppants into anopening in the formation to aid propagation of a fracture. The pressureof the injecting fluid causes the formation to fracture, and while thefluid is allowed to flow back to the surface, the proppants remain inthe fracture and prevent the formation from closing or collapsing.Conventionally, CO₂-based fracturing fluids have such a low viscositythat proppant particles added to the fluid immediately sink and are notpumped into the fracture until the last of the fluid has been used. As aresult, the fracture may close prematurely.

Embodiments of the present disclosure address these difficulties byproviding self-suspending proppant particles for use in CO₂-based fluidsand simplified, economical methods for treating subterranean formationswith hydraulic fracturing fluids containing the self-suspending proppantparticles suspended in a CO₂-based fluid. The self-suspending proppantsare proppant particles coated with a CO₂-philic material that is lightlycrosslinked and has a physical structure that can constrain CO₂molecules. As used throughout the disclosure, “CO₂-philic” refers to amolecule, ion, polymer, or composition having an affinity or attractionto CO₂. The term “CO₂-based fluid” refers to a fluid that includes CO₂.

Specific embodiments will now be described with references to thefigures. Whenever possible, the same reference numerals will be usedthroughout the drawings to refer to the same or like parts. As usedthroughout this disclosure, the singular forms “a,” “an” and “the”include plural referents unless the context clearly dictates otherwise.Thus, for example, reference to “a” component includes aspects havingtwo or more such components, unless the context clearly indicatesotherwise.

FIG. 1 schematically portrays three states of a proppant particle 110.On the left, an uncoated proppant particle 110 is depicted in a first,uncoated state. Then, in the middle, a coated proppant particle 142 isdepicted in which the proppant particle 110 is in a second, coatedstate. Finally, on the right is a swollen self-suspending proppant 140,in which the proppant particle 110 is in a third, swollen state. In thefirst state, the proppant particle 110 is not coated and, when placed ina CO₂-based fluid, it would likely sink to the bottom of the fluid onaccount of the low viscosity of CO₂-based fluids. In the second state,the proppant particle 110 has undergone a coating step 310 to be coatedwith a CO₂-philic coating 120, forming a coated proppant particle 142.In the third state, the coated proppant particle 142 has undergone acontacting step 320 to be contacted with CO₂ molecules, forming aswollen self-suspending proppant 140 that is coated with a now swollenCO₂-philic coating 121. Upon contact with CO₂ molecules during thecontacting step 320, the CO₂-philic coating 120 has volumetricallyexpanded to a swollen state, transforming the coated proppant particle142 into a swollen self-suspending proppant 140.

As illustrated by FIG. 1, in some embodiments, the coated proppantparticle 142 and the swollen self-suspending proppant 140 both includeat least one proppant particle 110 coated with a CO₂-philic coating 120,121. The proppant particle 110 may be chosen from any type of proppantsuitable for use in hydraulic fracturing applications. As previouslydescribed, proppants are propping agent particles used in hydraulicfracturing fluids to maintain and hold open subterranean fracturesduring or following subterranean treatment. In some embodiments, theproppant particle 110 may comprise particles of materials such asoxides, silicates, sand, ceramic, resin, plastic, mineral, glass, orcombinations thereof. For instance, the proppant particle 110 maycomprise graded sand, treated sand, resin-coated sand, ceramicproppants, plastic proppants, low-density proppants, or otherresin-coated materials. The proppant particle 110 may comprise particlesof bauxite or of sintered bauxite. The proppant particle 110 maycomprise glass particles or glass beads. Embodiments of the presentdisclosure may utilize at least one proppant particle 110 and inembodiments in which more than one proppant particle 110 is used, theproppant particles 110 may contain a mixture of two or more differentmaterials or three or more different materials.

The material of the proppant particle 110 may be chosen based on theparticular application and characteristics desired in a swollenself-suspending proppant 140. For instance, ceramic proppant materialsmay be suitable in embodiments desiring high strength, uniform size andshape, high thermal resistance and high conductivity. Fully or partiallycured resin-coated sand may be chosen in embodiments to provideparticles of irregular size and shape with medium crush resistancestrength and medium conductivity. Sands may be chosen in embodimentsdesiring naturally occurring and cost effective proppants or lowparticle strength and low conductivity.

The proppant particle 110 may have any size and shape. In someembodiments, the one or more proppant particles 110 may have sizes from8 mesh to 140 mesh (diameters from 106 micrometers (μm) to 2.36millimeters (mm)). In some embodiments, the proppant particles 110 mayhave sizes from 16 mesh to 30 mesh (diam. 600 μm to 1180 μm), 20 mesh to40 mesh (diam. 420 μm to 840 μm), 30 mesh to 50 mesh (diam. 300 μm to600 μm), 40 mesh to 70 mesh (diam. 212 μm to 420 μm) or 70 mesh to 140mesh (diam. 106 μm to 212 μm). The sphericity and roundness of theproppant particles 110 may also vary based on the desired application.

In some embodiments, the proppant particles 110 may have a rough surfacethat may increase adhesion of the CO₂-philic coating 120 to the proppantparticle 110 and may increase interaction of CO₂ with the CO₂-philiccoating 120. The proppant particles 110 may be roughened to increase thesurface area of the proppant particle 110 by any suitable physical orchemical method, including, for example, using an appropriate etchant.In some embodiments, the proppant particle 110 may have a surface thatprovides a desired adherence of the CO₂-philic coating 120 to theproppant particle 110 or may already be sufficiently rough without aneed for chemical or physical roughening.

The term “rough” refers to a surface having at least one deviation fromthe normalized plane of the surface, such as a depression or protrusion.The surface may be uneven and irregular and may have one or moreimperfections, such as dimples, stipples, bumps, projections or othersurface defects. The rough surface may have an arithmetic averageroughness (R_(a)) of greater than or equal to 1 nanometer (nm) (0.001μm). R_(a) is defined as the arithmetic average of the differencesbetween the local surface heights and the average surface height and canbe described by Equation 1, contemplating n measurements:

$\begin{matrix}{R_{a} = {\frac{1}{n}{\sum\limits_{i = 1}^{n}{y_{i}}}}} & {{EQUATION}\mspace{14mu} 1}\end{matrix}$

In Equation 1, each y_(i) is the amount of deviation from the normalizedplane of the surface (meaning the depth or height of a depression orprotrusion, respectively) of the absolute value of the ith of nmeasurements. Thus, R_(a) is the arithmetic average of the absolutevalues of n measurements of deviation y from the normalized plane of thesurface. In some embodiments, the surface of the proppant particle 110may have an R_(a) of greater than or equal to 2 nm (0.002 μm), orgreater than or equal to 10 nm (0.01 μm), or greater than or equal to 50nm (0.05 μm), or greater than or equal to 100 nm (0.1 μm), or greaterthan or equal to 1 μm.

As previously described, “CO₂-philic” refers to a molecule, ion,polymer, or composition having an affinity or attraction to CO₂. ACO₂-philic material possesses a tendency to mix with, dissolve in, or bewetted by CO₂. In some embodiments, the CO₂-philic material may beattracted to CO₂ molecules through intermolecular forces such as van derWaals forces or through hydrogen bonding. The CO₂-philic coating 120 maybe any coating formed from a material or combination of materials thathas a chemical or physical affinity to CO₂. The physical affinity of theCO₂-philic coating 120 may include, for example, being capable ofabsorbing or adsorbing CO₂ molecules.

In some embodiments, the CO₂-philic coating 120 may include or mayconsist of oxygen-containing molecules such as polysaccharide acetates,polyethylene glycols, ethylene glycol-containing polymers, partiallyfluorinated oxygen-containing polymers, oxygenated polymers, crosslinkedoxygen-containing polystyrenes, polyvinyl acetates, or combinationsthereof. For instance, in some embodiments, the CO₂-philic coating 120may be a polysaccharide acetate, such as cellulose acetate. TheCO₂-philic coating 120 in some embodiments may include CO₂-philicmaterials having silicon-containing groups. Examples of CO₂-philicmaterials having silicon-containing groups include polysiloxanes, suchas those which include monomeric units such as alkylsiloxanes,fluoroalkylsiloxanes, or chloroalkylsiloxanes. Examples of suitablealkylsiloxanes include dimethyl siloxanes and polydimethylsiloxanes. TheCO₂-philic coating 120 may comprise halogen compounds (such asfluorine-containing compounds) or compounds having halogenated carbons(for example, fluorocarbons). The CO₂-philic coating 120 may comprisebranched polyalkylene oxides or fluorinated polyethers, for example. Insome embodiments, the CO₂-philic coating 120 may comprise afluoropolymer. In some embodiments, the CO₂-philic coating 120 maycomprise ethylene glycol-containing polymers made from di(ethyleneglycol) monomethyl ether methacrylate or poly(ethylene glycol) methylether methacrylate crossklinked with ethylene glycol dimethacrylate ordi(ethylene glycol) dimethacrylate (DEGDMA).

In some embodiments, the CO₂-philic coating 120 may be formed frommonomeric materials or from oligomeric materials such as, for example,oligomers having 8 or fewer monomer units. The CO₂-philic coating 120may be an oligomer containing less than or equal to 8 repeatingmonomers, such as α or fewer repeating monomers or 3 or fewer repeatingmonomers. In some embodiments, the CO₂-philic coating 120 may be afluoride-containing oligomer. The CO₂-philic coating 120 may be afluoride-containing oligomer having 8 or fewer repeating monomer units,or 5 or fewer repeating monomer units, or 3 or fewer monomer units. Thefluoropolymers may be formed from monomers including, as non-limitingexamples, fluoroacrylate monomers such as2-(N-ethylperfluorooctane-sulfonamido) ethyl acrylate (“EtFOSEA”),2-(N-ethylperfluorooctane-sulfonamido) ethyl methacrylate (“EtFOSEMA”),2-(N-methylperfluorooctane-sulfonamido) ethyl acrylate (“MeFOSEA”),2-(N-methylperfluorooctane-sulfonamido) ethyl methacrylate (“MeFOSEMA”),1,1′-dihydroperfluorooctyl acrylate (“FOA”), 1,1′-dihydroperfluorooctylmethacrylate (“FOMA”), 1,1′,2,2′-tetrahydroperfluoroalkylacrylates,1,1′,2,2′-tetrahydroperfluoroalkyl-methacrylates and otherfluoromethacrylates; fluorostyrene monomers such as α-fluorostyrene and2,4,6-trifluoromethylstyrene; fluoroalkylene oxide monomers such ashexafluoropropylene oxide and perfluorocyclohexane oxide; fluoroolefinssuch as tetrafluoroethylene, vinylidine fluoride, andchlorotrifluoroethylene; and fluorinated alkyl vinyl ether monomers suchas perfluoro(propyl vinyl ether) and perfluoro(methyl vinyl ether).

The CO₂-philic coating 120 may comprise a polyether-based polymerincluding, but not limited to polyethers substituted with at least oneside group, which may include one or more groups that interact favorablywith or has an affinity for CO₂ (such as a Lewis base group), apoly(ether-carbonate), a poly(ether-carbonate) substituted with at leastone side group including a Lewis base, a vinyl polymer substituted withat least one side group including a Lewis base, a poly(ether-ester) or apoly(ether-ester) substituted with at least one side group including aLewis base. One possible non-limiting example of a Lewis base is anamino functional group.

While embodiments of suitable CO₂-philic materials were described mainlywith reference to the CO₂-philic coating 120, it should be understoodthat the CO₂-philic materials are equally applicable to the swollenCO₂-philic coating 121, which may be in accordance with any of theembodiments of CO₂-philic materials previously described.

In some embodiments, the CO₂-philic coating 120 may have hydrophobictendencies, such as a lack of attraction to water, repulsion to water,or immiscibility in water. The CO₂-philic coating 120 may notsubstantially dissolve (does not dissolve more than 10 weight percent(wt. %) or more than 8 wt. %, or more than 5 wt. % or more than 3 wt. %)when contacted with, submerged in, or otherwise exposed to water. Insome embodiments, the CO₂-philic coating 120 may not dissociate from theproppant particle 110 when the self-suspending proppant 140 is added toa water-based fluid, such as water or a fluid that includes water. Insome embodiments, the CO₂-philic coating 120 of a coated proppantparticle 142 or the swollen CO₂-philic coating 121 of a swollenself-suspending proppant 140 does not dissociate from the proppantparticle 110 when the self-suspending proppant 140 is contacted,exposed, or placed in a fluid medium having at least 20 wt. % waterbased on the total weight of the fluid medium. In other embodiments, theCO₂-philic coating 120 of a coated proppant particle 142 or the swollenCO₂-philic coating 121 of a swollen self-suspending proppant 140 doesnot substantially dissociate (does not dissipate more than 10 wt. %, ormore than 8 wt. %, or more than 5 wt. % or more than 3 wt. %) from theproppant particle 110 when the self-suspending proppant 140 iscontacted, exposed, or placed in a fluid medium having at least 30 wt. %water, or at least 50 wt. % water, or at least 75 wt. % water.Dissolution of the CO₂-philic coating in a fluid medium may bedetermined by any suitable analytical technique for detection ofsolvated coating material that is performed on a fluid medium to which acoated proppant particle 142 has been added and allowed to equilibrateat room temperature for at least 24 hours.

Similarly, the CO₂-philic coating 120 may repel water, may not beattracted to water, or may not be miscible in water. Therefore, in someembodiments, the CO₂-philic coating 120 of a coated proppant particle142 may not swell when the coated proppant particle 142 is added to awater-based fluid, such as water or a fluid containing water. In someembodiments, the CO₂-philic coating 120 of a coated proppant particle142 may not swell when the coated proppant particle 142 is contactedwith, exposed to, or placed in a fluid medium having at least 20 wt. %water based on the total weight of the fluid medium. In otherembodiments, the CO₂-philic coating 120 may not swell when the coatedproppant particle 142 is present in a fluid medium having at least 30wt. % water, or at least 50 wt. % water, or at least 75 wt. % water.

As a non-limiting example, a batch of coated proppant particles 142having the same CO₂-philic coating 120 is tested in which half of thebatch is added to a CO₂-based fluid 130 and the other half of the batchis added to water. The CO₂-philic coating 120 of the coated proppantparticles 142 added to the CO₂-base fluid exhibit a swelling (asmeasured based on the volumetric expansion of the particles before andafter addition to the fluid after 30 minutes, as previously described)of at least 2 times, at least 3 times, at least 5 times, at least 50times, or at least 100 times the amount of the swelling the coatedproppants 142 added to water exhibit. In some embodiments, when added towater, the CO₂-philic coating 120 of a coated proppant particle 142 mayswell less than or equal to 1/10 the amount the CO₂-philic coating 120of a coated proppant particle 142 swells when added to a CO₂-based fluid130. In some embodiments, the CO₂-philic coating 120 of a coatedproppant particle 142 when added to water may swell less than or equalto ½, or less than or equal to ⅓, or less than or equal to 1/100, orless than or equal to 1/1,000 the amount the CO₂-philic coating 120 of acoated proppant particle 142 swells when added to a CO₂-based fluid 130.

Referring again to FIG. 1, in one or more embodiments, the proppantparticle 110 may be coated with a CO₂-philic coating 120 during acoating step 310 to produce, form, or result in a swollenself-suspending proppant 140. In some embodiments, the CO₂-philiccoating 120 may be a surface layer on or bound to the proppant particle110. Such a surface layer may cover at least a portion of the surface ofthe proppant particle 110. For example, the CO₂-philic coating 120 maycoat, overlay, enclose, envelop, or otherwise surround the proppantparticle 110 with the coating. The CO₂-philic coating 120 may cover theentire surface of the proppant particle 110 (as shown) or,alternatively, may only partially surround the proppant particle 110(not shown), leaving at least a portion of surface of the proppantparticle 110 uncoated or otherwise exposed.

The CO₂-philic coating 120 in some embodiments may be lightlycrosslinked. As used throughout this disclosure, “lightly crosslinked”refers to partial crosslinking, meaning that at least one crosslinkablesite is not crosslinked. For some embodiments described throughout thisdisclosure, a “lightly crosslinked” CO₂-philic coating 120 may exhibitvolumetric swelling of at least 25% at room temperature in the presenceof CO₂-based fluid to form the swollen CO₂-philic coating 121. Withoutintent to be bound by theory, it is believed that greater degrees ofcrosslinking in the CO₂-philic coating 120 may result in a maximumvolumetric swelling of the swollen CO₂-philic coating 121 of less than25% at room temperature in the presence of CO₂-based fluid. Furthermore,it is believed that in some highly crosslinked polymeric CO₂-philicmaterials, the rigidity of the polymer segments between crosslinkjunctions are short and inflexible, such that swelling does not occurwhen carbon dioxide solvates the polymer segments of such materials. Thedegree of crosslinking may be controlled by the molar or weight ratio ofcrosslinkers to monomers used as reactants for forming the CO₂-philiccoating 120. A suitable degree of crosslinking may be determinedempirically by measuring degrees of swelling in different CO₂-basedfluids as a function of temperature and pressure for polymers preparedwith varying crosslinker to monomer ratios. Additionally, highlycrosslinked coatings may not exhibit an optimal physical structure forconstraining or accommodating CO₂ and, therefore, may not exhibitoptimal volumetric expansion from the non-swollen state to the swollenstate. In contrast, a lightly crosslinked swollen CO₂-phillic coating121 may retain its shape without dissolving in the fluid system, whilemaintaining a sufficient attraction or bond to the proppant particle110.

In some embodiments, a lightly crosslinked CO₂-philic coating 120 mayconstrain CO₂ molecules. In some embodiments, the light crosslinking maycreate a polymeric network having spaces or voids that accommodate CO₂molecules. Without intent to be bound by theory, it is believed that thephysical structure of the coating may draw in CO₂ as a result of anattraction between the CO₂-philic coating 120 and CO₂ molecules to formthe swollen CO₂-philic coating 121. As the CO₂ molecules are drawn intothe CO₂-philic coating 120, the CO₂-philic coating 120 may constrain orbe solvated with one or more CO₂ molecules to form the swollenCO₂-philic coating 121. In turn, the coated proppant particle 142 mayundergo a volumetric expansion from a non-swollen to a swollen state,resulting in a swollen self-suspending proppant 140. Constraint of oneor more CO₂ molecules may occur by sorption, for example. Sorption mayinclude physical or chemical adsorption of the CO₂ molecules, physicalor chemical absorption of the CO₂ molecules, or any combination ofthese.

As used throughout this disclosure, “volumetric expansion (E)” refers tothe difference of the volume (V₁) of a particular number of coatedproppant particles in the swollen state and the volume (V₀) of the samenumber of coated proppant particles in the non-swollen state, divided bythe volume (V₀) of the coated proppant particles in the non-swollenstate in accordance with Equation 2:

E=(V ₁ −V ₀)/(V ₀)  EQUATION 2

To express volumetric expansion as a percent, E may be multiplied by100. The respective volumes of a coated proppant particle 142 (in thenon-swollen state) and the volume of a swollen self-suspending proppant140 (in the swollen state) include the combined volume of both theproppant particle 110 and the CO₂-philic coating 120.

In some embodiments, the coated proppant particle 142 may volumetricallyexpand from a non-swollen state to a swollen state. The CO₂-philiccoating 120 of the coated proppant particle 142 may volumetricallyexpand when CO₂ is constrained within the physical structure of thecoating, forming a swollen CO₂-philic coating 121 resulting in a swollenself-suspending proppant 140. In some embodiments, the swollenself-suspending proppant 140 may exhibit a volumetric expansion of atleast 100% from a non-swollen state to a swollen state. In someembodiments, the swollen self-suspending proppant 140 may volumetricallyexpand at least 10%, at least 15%, at least 20%, at least 25%, at least50%, at least 200%, or at least 300% from the non-swollen state to theswollen state. Without being bound by theory, such an expansion mayincrease the buoyancy of the swollen self-suspending proppant 140 andmay facilitate or give rise to the suspension of particles in CO₂-basedfluids 130, as shown in FIG. 2B.

In some embodiments, at least in part because of this volumetricexpansion, the density of the swollen self-suspending proppants 140 maybe less than, equal to, or only slightly greater than the density of theCO₂-based fluid 130. In some embodiments, the swollen self-suspendingproppant 140 may have a density of less than or equal to 10% of thedensity of the CO₂-based fluid 130, for example. The density of theswollen self-suspending proppant 140 may be less than or equal to 50%,or less than or equal to 70%, or less than or equal to 85%, or less thanor equal to 90% of an unswollen proppant. In some embodiments, one ormore viscosifiers may be added to the CO₂-based fluid 130, such that thedensity of the swollen self-suspending proppant 140 may be less than orequal to 100%, or less than or equal to 125%, or less than or equal to150%, or less than or equal to 200%, or less than or equal to 250%, orless than or equal to 300% of the density of the CO₂-based fluid 130.The density of the swollen self-suspending proppant 140 may be from 25%to 200% of the density of the CO₂-based fluid 130. In some embodiments,the density of the swollen self-suspending proppant 140 may be from 25%to 125%, or from 50% to 200%, or from 50% to 125%, or from 75% to 150%,or from 75% to 200%, or from 25% to 100%, or from 25% to 110% of thedensity of the CO₂-based fluid 130. The difference in density may causethe swollen self-suspending proppants 140 to suspend in the CO₂-basedfluid 130 without sinking to the bottom or otherwise coalescing in thefluid. The difference in density may reduce or eliminate the need foradditional viscosifiers to be added to the CO₂-based fluid 130.

As previously described, a “CO₂-based fluid” refers to a fluid thatincludes CO₂. In some embodiments, the CO₂-based fluid 130 may compriseat least 30 wt. % CO₂, based on the total weight the CO₂-based fluid 130before the CO₂-based fluid 130 is contacted with a swollenself-suspending proppant 140. In some embodiments, the CO₂-based fluid130 may comprise at least 45 wt. % or at least 51 wt. % CO₂. In someembodiments, the CO₂-based fluid 130 may comprise at least 60 wt. %, orat least 75 wt. %, or at least 85 wt. %, or at least 90 wt. % CO₂. Insome embodiments, the CO₂-based fluid 130 may comprise liquid CO₂ orgaseous CO₂. In some embodiments, the CO₂-based fluid 130 may comprisesupercritical, subcritical, or critical CO₂.

As used throughout this disclosure, “supercritical” refers to asubstance at a pressure and a temperature greater than those of thesubstance's critical point, such that distinct phases do not exist andthe substance may exhibit the diffusion of a gas while dissolvingmaterials like a liquid. Similarly, “subcritical” refers to a substancewith a pressure and a temperature of less than those of the substance'scritical point, such that distinct phases exist. Likewise, “critical”refers to a substance that is at a pressure and temperature equal tothose of the substance's critical point. In some embodiments, theCO₂-based fluid 130 may be supercritical CO₂. In some embodiments, theCO₂-based fluid 130 may be liquid CO₂. In some embodiments, theCO₂-based fluid 130 may include substances other than CO₂. The CO₂-basedfluid 130 may contain additional fluids or gasses, including but notlimited to N₂, dimethyl ether, or hydrocarbons.

Still referring to FIG. 1, further embodiments of the present disclosureare directed to methods for producing self-suspending proppants 140,142. In some embodiments, the method may include a coating step 310 inwhich a proppant particle 110 is coated with a monomeric precursormaterial capable of forming a CO₂-philic material upon polymerization toform a self-suspending proppant. The precursor material may includemonomers and, optionally one or more crosslinking agents. In someembodiments, as previously discussed, the degree of crosslinking may becontrolled by the molar or weight ratio of crosslinkers to monomers toachieve a coating that is lightly crosslinked.

In some embodiments, the coating step 310 may include contacting thepolymerizable precursor material with the proppant particle 110 in afluidized bed process. In some embodiments, the coating step 310 mayinclude a stationary, bubbling, circulation, or vibratory fluidized bedprocess. In some embodiments, the coating step 310 may include sprayingor saturating the proppant particles 110 with a CO₂-philic polymer orpolymer precursor. The coating step 310 may include, in someembodiments, tumbling or agitating the coated proppant to preventagglomeration or clumping. The coating step 310 may include mixing aCO₂-philic material with another compound such as, for example, asolvent, an initiator, an adhesion promoter, or an additive, to form theCO₂-philic coating 120. In some embodiments, the coating process may beconducted in an emulsion coating technique. In some embodiments, theadhesion promoter may comprise a silane (for example, amino silane) or asilane-containing monomer. In some embodiments, an adhesion promoter maynot be necessary to coat the proppant particles 110.

The methods for producing a self-suspending proppant may includepolymerizing a polymerizable precursor material that has been coatedonto the proppant particle 110 during the coating step 310. In someembodiments, the polymerizable precursor materials may incorporate apolymerization initiator compound. In some embodiments, polymerizing thepolymerizable precursor material may include heating the coated proppantto a temperature sufficient to polymerize at least a portion of theprecursor material. The heating may include application of heat by anysuitable process such as by forced hot air heating, convection,friction, conduction, combustion, exothermic reactions, microwaveheating, or infrared radiation, for example. The coated proppantparticle may be heated at a polymerization temperature and for apolymerization time sufficient to crosslink at least a portion of thepolymerizable precursor material. In some embodiments, thepolymerization step may include subjecting the coated proppant toultraviolet (UV) light or any other polymerization techniques known inthe art. The precursor material may be polymerized at conditionssufficient to crosslink at least a portion of the precursor material.

In some embodiments, the methods for producing a self-suspendingproppant may further comprise roughening the proppant particles 110before the coating step 310. The proppant particles 110 may bechemically or physically roughened, as previously described. In someembodiments, roughening the proppant particles 110 may produce roughparticle surfaces with an arithmetic average roughness (R_(a)) ofgreater than or equal to 0.001 μm (1 nm). In some embodiments, the roughparticle surfaces may have an R_(a) of greater than or equal to 0.002 μm(2 nm), 0.005 μm (5 nm), or greater than or equal to 0.01 μm (10 nm), orgreater than or equal to 0.05 μm (50 nm), or greater than or equal to0.1 μm (100 nm), or greater than or equal to 0.5 μm, or greater than orequal to 1 μm when measured as previously discussed.

The method for producing a self-suspending proppant may include coatingthe proppant particle 110 using a two-layer coating or multi-layeredcoating system. The method in some embodiments may further includecoating proppant particles 110 with a top coating. The top coating maybe an overlying layer that may be added for additional properties orfeatures. As a non-limiting example, additional coatings may be used inconjunction with, or may comprise a breaker. As used throughout thisdisclosure, a “breaker” refers to a compound that may break or degradethe polymers of the coating after a fracturing operation to preventformation damage. In some embodiments, the breaker may be an oxidizer orenzyme breaker. The breaker may be any suitable materials capable ofdegrading a coating material.

Referring to FIG. 2A, a container 160 of conventional hydraulicfracturing fluid 152 is schematically illustrated to show how uncoatedproppant particles 110 react when added to a CO₂-based fluid 130. Theconventional hydraulic fracturing fluid 152 includes uncoated proppantparticles 110 in a CO₂-based fluid 130. As shown, the uncoated proppantparticles 110 sink to the bottom of the container 160, as theconventional hydraulic fracturing fluid 152 is not sufficiently viscousto support the particles without the aid of additional viscosifyingmeans. Settled proppant particles 110 may cause problems in hydraulicfracturing processes. Mitigation of these problems typically requiresuse of turbulence, or additions of viscosifiers or other ingredients tothe conventional CO₂-based hydraulic fracturing fluid 152 to ensure thatthe proppant particles 110 suspend in the CO₂-based fluid 130. Theseadditional required steps and products may increase the time and costsassociated with subterranean treatment.

In contrast, FIG. 2B depicts a container 160 of hydraulic fracturingfluid 150 according to the present embodiments. FIG. 2B schematicallyillustrates the effect of a coated proppant particle 142 (as shown inFIG. 1) once placed in CO₂-based fluid 130. Once placed in CO₂-basedfluid 130, as previously discussed, the CO₂-philic coating 120 (shown inFIG. 1) of the coated proppant particle 142 volumetrically expands toform a swollen self-suspending proppant 140. The hydraulic fracturingfluid 150 of the present disclosure, as shown in FIG. 2B, containsswollen self-suspending proppants 140 including proppant particles 110coated with a swollen CO₂-philic coating 121. Without being bound bytheory, the swollen self-suspending proppants 140 of the presentdisclosure may overcome the difficulties faced when using conventionalhydraulic fracturing fluid 152, as previously discussed, by stabilizinga suspension in hydraulic fluid through the ability of the CO₂-philiccoating 120 to volumetrically expand upon contact with CO₂ to result ina swollen CO₂-philic coating 121. The swollen self-suspending proppant140 may be in accordance with any of the embodiments previouslydescribed in which the swollen self-suspending proppant 140 comprises aproppant particle 110 that has been coated with a CO₂-philic coating 120and contacted with CO₂ to form the swollen CO₂-philic coating 121.

The hydraulic fracturing fluid 150 of FIG. 2B contains swollenself-suspending proppants 140 suspended in the CO₂-based fluid 130. Asdiscussed, in some embodiments, the CO₂-philic coating 120 may at leastpartially constrain (such as through sorption) CO₂ present in theCO₂-based fluid 130 to produce the swollen CO₂-philic coating 121. Theconstrained CO₂ molecules may cause the CO₂-philic coating 120, andthus, the swollen self-suspending proppants 140, to volumetricallyexpand to a swollen state. The swollen self-suspending proppants 140 maybecome less dense, more buoyant in the CO₂-based fluid, or both, as aresult of the constrained CO₂ molecules in the swollen CO₂-philiccoating 121. The decreased density or increased buoyancy of the swollenself-suspending proppants 140 may facilitate or enable the particles toself-suspend in the CO₂-based fluid 130.

Embodiments of methods for preparing a hydraulic fracturing fluid 150may include contacting the coated proppant particle 142 of any of theembodiments previously discussed with a CO₂-based fluid 130 containingCO₂ to form a hydraulic fracturing fluid 150. The hydraulic fracturingfluid 150 may be in accordance with any of the embodiments previouslydescribed. Upon contact with the CO₂-based fluid 130 the CO₂-philiccoating 120 of the coated proppant particle 142 may swell to a swollenCO₂-philic coating 121, producing a swollen self-suspending proppant140. The swollen self-suspending proppants 140 may suspend within thehydraulic fracturing fluid 150 without any need for, or with a reducedneed for, additives such as viscosifiers. However, the hydraulicfracturing fluid 150 in some embodiments may contain any additivescommonly used in the art of fracking, including viscosifiers, forexample. The hydraulic fracturing fluid 150 may contain biocides,breakers, buffers, stabilizers, diverting agents, fluid loss additives,friction reducers, iron controllers, surfactants, gel stabilizers, andviscosifiers. In some embodiments, the hydraulic fracturing fluid 150may not contain any additional viscosifiers. In some embodiments, thehydraulic fracturing fluid 150 may include less than or equal to 30 wt.%, or 20 wt. %, or 15 wt. % or 10 wt. % of any additional viscosifiers.In an embodiment, fracturing fluid having CO₂ and hydrocarbons maycontain viscosifiers for hydrocarbons such as alkyl phosphonates, incombination with iron (3+) or aluminum (3+) crosslinkers.

FIG. 3 is a schematic diagram of a subterranean formation being treatedusing hydraulic fracturing fluid containing proppants to propagate afracture. As previously discussed, suspended proppant particles 110within a hydraulic fracturing fluid 150 may aid in treating subterraneanfractures, to prop open and keep open the fracture. FIG. 3 shows asubterranean fracture 260 that has been injected with hydraulicfracturing fluid 150 in accordance with the present embodimentscontaining swollen self-suspending proppants 140, according to theembodiments previously shown and described.

The subterranean fracture 260 of FIG. 3 was generated by contacting asubterranean formation with hydraulic fracturing fluid 150 and using thehydraulic fracturing fluid 150 to propagate and further open thesubterranean fracture 260. The hydraulic fracturing fluid 150 in thesubterranean fracture 260 of FIG. 3 comprises swollen self-suspendingproppants 140 suspended in a CO₂-based fluid 130. In some embodiments,the swollen self-suspending proppants 140, owing in part to theirincreased buoyancy, may be distributed throughout the CO₂-based fluid130. The swollen self-suspending proppants 140, as discussed previously,may have a density or buoyancy that prevents the particles fromaggregating or otherwise coalescing within the subterranean fracture260, owing in part to the swollen CO₂-philic coating 121. The hydraulicfracturing fluid 150 may be pumped into the subterranean fracture 260 ormay be otherwise contacted with the subterranean formation. In someembodiments the hydraulic fracturing fluid 150 may be pressurized.

Embodiments of methods of treating a subterranean formation may includecontacting the subterranean formation with the hydraulic fracturingfluid 150 that includes swollen self-suspending proppants 140 and aCO₂-based fluid 130 in accordance with any of the embodiments previouslydiscussed. Such methods of treating a subterranean formation may includepropagating at least one subterranean fracture 260 in the subterraneanformation to treat the subterranean formation. In some embodiments, thesubterranean formation may be a rock or shale formation. In someembodiments, contacting of the subterranean formation may includedrilling into the subterranean formation and subsequently injecting thehydraulic fracturing fluid 150 into at least one subterranean fracture260 in the subterranean formation. In some embodiments, the hydraulicfracturing fluid 150 may be pressurized before being injected into thesubterranean fracture 260 in the subterranean formation.

Though embodiments of the present disclosure have been discussed in thecontext of hydraulic fracturing processes, embodiments of the presentdisclosure may also be used in other industries. For example, in someembodiments, the swollen self-suspending proppants 140 and hydraulicfracturing fluid 150 of the present disclosure may be used to stimulategroundwater wells, to precondition or induce rock cave-ins for miningoperations, to dispose of waste by injecting it deeply into rock, tomeasure stresses in the Earth's crust, to generate electricity inenhanced geothermal systems, or to increase injection rates for thegeologic sequestration of CO₂.

It should be apparent to those skilled in the art that variousmodifications and variations may be made to the embodiments describedwithin without departing from the spirit and scope of the claimedsubject matter. Thus, it is intended that the specification cover themodifications and variations of the various embodiments described withinprovided such modification and variations come within the scope of theappended claims and their equivalents.

Having described the subject matter of the present disclosure in detailand by reference to specific embodiments thereof, it is noted that thevarious details disclosed within should not be taken to imply that thesedetails relate to elements that are essential components of the variousembodiments described within, even in cases where a particular elementis illustrated in each of the drawings that accompany the presentdescription. Further, it should be apparent that modifications andvariations are possible without departing from the scope of the presentdisclosure, including, but not limited to, embodiments defined in theappended claims. More specifically, although some aspects of the presentdisclosure are identified as particularly advantageous, it iscontemplated that the present disclosure is not necessarily limited tothese aspects.

1. A self-suspending proppant comprising proppant particles coated witha CO₂-philic coating, in which the CO₂-philic coating is lightlycrosslinked and has a physical structure that constrains CO₂ molecules.2. The self-suspending proppant of claim 1, in which the self-suspendingproppant volumetrically expands from a non-swollen state to a swollenstate when CO₂ molecules are constrained within the physical structure.3. (canceled)
 4. The self-suspending proppant of claim 2, in which thephysical structure of the CO₂-philic coating volumetrically expands byat least 100% from the non-swollen state to the swollen state. 5.(canceled)
 6. The self-suspending proppant of claim 1, in which theproppant particles have a rough surface with an arithmetic averageroughness (R_(a)) of greater than or equal to 1 nanometer (nm).
 7. Theself-suspending proppant of claim 1, in which the CO₂-philic coatingcomprises a CO₂-philic material chosen from polysaccharide acetates,polyethylene glycols, partially fluorinated oxygen-containing polymers,oxygenated polymers, crosslinked oxygen-containing polystyrenes,polyvinyl acetates, and combinations of any of these.
 8. (canceled) 9.The self-suspending proppant of claim 1, in which the CO₂-philic coatingdoes not dissociate from the proppant particles when the self-suspendingproppant is added to a water-based fluid.
 10. A hydraulic fracturingfluid comprising: a fluid comprising CO₂, and self-suspending proppantssuspended in the fluid, the self-suspending proppants comprisingproppant particles coated with a CO₂-philic coating in which theCO₂-philic coating is lightly crosslinked and has a physical structurethat constrains CO₂ molecules.
 11. (canceled)
 12. The hydraulicfracturing fluid of claim 10, in which the fluid further comprises N₂.13. The hydraulic fracturing fluid of claim 10, in which the physicalstructure of the CO₂-philic coating comprises constrained CO₂ moleculesthat volumetrically expand the self-suspending proppants to a swollenstate.
 14. (canceled)
 15. The hydraulic fracturing fluid of claim 13, inwhich the physical structure of the CO₂-philic coating is volumetricallyexpanded by at least 100% from the non-swollen state to the swollenstate. 16-17. (canceled)
 18. The hydraulic fracturing fluid of claim 10,in which the self-suspending proppants in the swollen state have adensity of less than or equal to 200% of the density of the fluid. 19.The hydraulic fracturing fluid of claim 10, in which the proppantparticles have rough surfaces with an arithmetic average roughness(R_(a)) of greater than or equal to 1 nm.
 20. The hydraulic fracturingfluid of claim 10, in which the CO₂-philic coating comprises aCO₂-philic material chosen from polysaccharide acetates, polyethyleneglycols, partially fluorinated oxygen-containing polymers, oxygenatedpolymers, crosslinked oxygen-containing polystyrenes, polyvinylacetates, and combinations of any of these.
 21. (canceled)
 22. Thehydraulic fracturing fluid of claim 10, in which the CO₂-philic coatingdoes not dissociate from the proppant particles when the self-suspendingproppants are added to a water-based fluid.
 23. A method for producingself-suspending proppant particles, the method comprising: coatingproppant particles with a polymerizable precursor material of aCO₂-philic material to form coated proppant particles; and polymerizingthe polymerizable precursor material to convert the coated proppantparticles to self-suspending proppant particles coated with theCO₂-philic material in which the CO₂-philic coating is lightlycrosslinked and has a physical structure that constrains CO₂ molecules.24. The method of claim 23, in which coating the proppant particlescomprises contacting the polymerizable precursor material with theproppant particles in a fluidized bed process.
 25. The method of claim23, further comprising coating the proppant particles with an adhesionpromoter before coating the proppant particles with the polymerizableprecursor material of a CO₂-philic material.
 26. The method of claim 23,further comprising roughening the proppant particles before the coatingstep, in which roughening the proppant particles produces proppantparticles having a rough surface with an arithmetic average roughness(R_(a)) of greater than or equal to 1 nm. 27-29. (canceled)
 30. Themethod of claim 23, in which the CO₂-philic material is chosen frompolysaccharide acetates, polyethylene glycols, partially fluorinatedoxygen-containing polymers, oxygenated polymers, crosslinkedoxygen-containing polystyrenes, polyvinyl acetates, and combinations ofany of these.
 31. (canceled)
 32. The method of claim 23, in which theCO₂-philic coating does not dissociate from the proppant particles whenthe self-suspending proppant particles are added to a water-based fluid.33-37. (canceled)